Crystal links ultraviolet photons

October 20/27, 2004

One key to enabling devices that store
and transmit information in individual particles like photons is controlling
entanglement.

When photons are entangled, aspects of the photons that can store
a bit of information -- like polarization -- remain linked regardless
of the physical distance between them. Entangled photons can be used in
quantum cryptography to allow for the exchange of perfectly secure cryptographic
keys. They can also be used to transfer information within quantum computers,
which are theoretically blazingly fast at certain very large problems,
including cracking today's encryption codes.

Researchers from Tohoku University in Japan have moved the quest
to control entanglement forward with a method that uses the energy from
a pair of photons to produce a pair of entangled photons that have the
same energy state as the original photons. The current method, parametric
downconversion, uses the energy from one photon to create a pair of lower-energy
entangled photons. Because the researchers' method does not halve the
energy of the original photons, it has the potential to entangle more
than two photons.

The researchers method calls for firing two ultraviolet photons
at a copper chloride crystal to produce a biexciton. An exciton is a pairing
of an electron and a hole where an electron is missing; a biexciton is
a pair of excitons. The biexciton decays into a pair of entangled ultraviolet
photons.

The use of a semiconductor material makes the method more amenable
to today's electronics, and could lead to electrically-driven sources
of entangled photons.

The method could be used for practical applications for entangled
photon sources in one to three years and current-driven entangled photon
sources in three to five years, according to the researchers. The work
appeared in the September 9, 2004 issue of Nature.